The present invention relates to data playback apparatus for recording or playing back information from a recording medium such as a compact disk, a digital audio tape and an optical video disk (which will hereinafter be referred to as CD, DAT and LD, respectively) on which recorded are general digital data such as still picture, computer program and character data concurrently with time-dependent data such as speech and motion picture.
Recently, as represented by a compact disk read-only memory (which will hereinafter be referred to as a CD-ROM) there are are established ways to record general data such as character data and computer program on a recording medium, and the corresponding playback apparatus are being used widely. One example of the data playback apparatus will be described hereinbelow.
A CD is for recording a two-channel audio which is converted into digital data under the conditions that the sampling frequency is 44.1 kHz and the number of the quantization bits is 16, and the transfer rate of audio data in reproduction is 176.4 kbyte/sec. On the other hand, a CD-ROM is for recording general data such as character data, image data and computer program instead of the audio data. The CD-ROM substantially has the same structure as the CD except recording contents. In the CD-ROM the audio data storing region is block-structured so as to record such data, thereby obtaining 150 kbyte/sec of transfer rate, as exemplified by "Electronics"1985 February P73-80.
There is a problem which arises with such an arrangement, however, in that a playback apparatus for the CD is directly applied for the CD-ROM to thereby obtain only 150 kbyte/sec of transfer rate. For instance, in the case of the transfer of image data, an image comprising 640.times.480 dots and having a data amount of 8 bits per pixel results in the data amount of 300 kbyte/sec per picture. That is, the read-out of the data corresponding to one picture requires 2 seconds.
A technique to improve the transfer rate is disclosed in U.S. Pat. No. 4,707,818 where the reduction of the transfer rate due to mixedly recording the audio data and the video data is minimized by increasing the reproduction linear velocity. That is, when mixedly recording both the audio data and video data on a CD, the transfer rate of the audio data is lowered so as to deteriorate the audio quality. In order to prevent the lowering of the transfer rate, the disk is rotated at a speed higher than the normal speed with respect to the region in which the audio data and video data are mixedly recorded. For the realization thereof, the reproduction linear velocity is recorded as control data in the disk and read out in reproduction of the data so as to perform the reproduction of the disk at a designated speed.
However, in the case of recording data in the CD-ROM in accordance with such a technique, difficulty is encountered to reproduce the data by means of a general CD-ROM drive unit. That is, because the real-time reproduction of the speech for this disk can be ensured only under the condition of increasing the linear velocity, it is difficult to perform the reproduction by means of a general CD-ROM drive unit which can realize only the transfer rate of 150 kbyte/sec.
In addition, since this unit is for directly processing and outputting the reproduced data on the basis of the real time and the reproduction velocity is determined in accordance with each of the data, difficulty is encountered to change the reproduction velocity in accordance with the destination of the data.